Introduction
There are two ways to find out how old a tree is. The first is to wait until the tree is cut down and count the rings. The second is to use an increment borer and count the rings on a trunk cross section. The study of tree rings to calculate a trees' age is called dendrochronology.

In each method the tree sustains damage. To obtain a trunk cross section sometimes called 'beaver cookies', 'tree cookies', or 'hockey pucks', the tree must be killed. To get an accurate age of the tree, the rings must be near the base (ground) of the tree. The borer takes a small (0.200 inch diameter) straw-like sample from the bark to the pith of the tree. Though this hole is small, it can still introduce decay in the trunk.

The increment borer is made of carbide steel which makes it a relatively expensive tool. The borer has three parts; the handle, the steel shaft and the extractor. Proper maintenance and operation are essential if the tool is to last. Borers come in different sizes and should be at least 75% the diameter of the tree you are boring. The instructions included with the increment borer provide many useful tips, and an expanded explanation that should be read before doing any boring.

Tree trunk growth results primarily from the xylem layers and is counted to determine the age. The tree produces large xylem cells in the spring and small cells of xylem in the summer, making it easy to note the difference between years. The tree rings are larger in wet years than in dry years. In some hardwood trees, the rings are so small that a hand lens and pins may be needed to count the rings. The size of the rings are now being used to determine past weather patterns, even with petrified trees. Trying to establish weather history is a very difficult process, and more than one or two samples of wood is needed.

The tree's age can therefore be obtained by counting the annual rings of either a 'core' or trunk cross section.

Students should get the data from two other groups and place the data in their data table.

Take the data from each year and divide by the total for all six years to find out what percentage of the total growth occurs each year. Place this data in Table B.

Table B: Percentage Growth - Tree Cross Section (mm.)

Sample

19__

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19__

19__

19__

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Discussion Questions

On which sample was it easier to count and measure the rings? List several reasons why you think there is this difference?

What year had the largest growth by ring width (in mm)?

Is the variation in tree cross section the same each year? What are some of the reasons that you can give for the differences / the similarities?

Conclusions

Does the data support your hypothesis? Explain.

What do the results of this experiment tell you about trees as a homeowner / forester?

What experiment could you design to tell more about the effects that cause changes in ring growth?

What conditions do you think caused the difference in ring size and color?

Background Information
The study of tree age using annual growth rings is called dendrochronology. Either tree cross sections or core samples from an increment borer can be used. The easiest way, is to cut a tree that has fallen, into sections. The drawback to this method is that the tree must die. To get an accurate measurement, the section must come from very near the base of the tree. Samples from different heights of a single tree can simulate trees of different ages for this laboratory.

The booklet with more information about the increment borer is The Care and Feeding of an Increment Borer by the USDA Forest Service. The increment borers range in price from $85 to over $150 depending on the size. A tree is normally cored no more than every 6 to 8 years. This method is used by foresters, on large tracks of commercial property to determine the age of a stand. If the tree's health is in doubt, a 'tree doctor' can use it to tell if their is internal damage. The cores, if done correctly, can even be used for chemical studies in commercial labs to determine suitability of the trees for different purposes. In the urban areas, city or state foresters may use the core to determine not just the age, but the structural condition of the tree. A hollow tree may be hazardous and have to be removed.

Target Group
Ninth through twelvth grades, but can be used for elementary through middle school with minor modifications.

Timeline
This exercise will normally take two hours to complete. A half-hour for an explanation of the lab, a half-hour for data gathering and the analysis by each group, a half-hour to get data from three other groups, and at least a half-hour to get into a class discussion of the differences among species or sites.

Placement of Lab in Curriculum
This laboratory could be used when discussing tree or plant growth and the patterns they exhibit. It could also help to explain the growth differences between species. This lab can be scheduled as a data gathering and analysis activity at any time of the year. If the lab is not used as a direct part of the Urban Forestry unit, it can serve as a rain / indoor activity. Lab-aids #52 (Lab-aids, Inc.) can be used as a pre-lab exercise for the coring and ring counting.

Student Learning Objectives

Evaluate and analyze the data collected to determine how trees change over time.

Replicate the results of another student's data.

Revise a prediction (hypothesis) based on additional data.

Identify appropriate methods of measurement for this task.

Preparation and Teaching Tips
If a Forester's Trunk is not available, prepare matching cores and 'tree cookies', from several trees, and a data sheet with all the information to complete the lab. Include a short discussion on what tree rings are, and how they may have additional meaning for us in reference to historical events. The data sheet will include age, species, and the type of site the tree was found on (forest, field, urban).

Become familiar with the terms associated with the cross sections of a tree or core. Have students look at the tree cross sections and form a hypothesis as to what species of tree they think it is.

Blowouts

Use microscopes or hand lenses to measure the differences in spring and summer wood. Compare three different species over the last four years and then back 20 years.

Compare twig growth on a live tree of the same species as one of the 'tree cookies' to compare growth during similar years.

Call or write to a Forester to find out how chemical data for trees would be used.

Create more questions about tree rings that could help homeowners know more about their trees.

Have the students determine the wood densities of the tree species. Then relate this information to the uses of different woods.